In conventional air purifiers and other such organic decomposition apparatuses that use photocatalysts, a porous substance supporting TiO2 or another photocatalytic material is irradiated with ultraviolet light emitted from a mercury lamp or the like, and the photocatalyst is excited. However, the mercury lamp must be placed separately from the substance. Therefore, the entire apparatus becomes large when, for example, an air purifier is involved.
A method for exciting the photocatalyst by using a surface emitter as a light source has been proposed (see Japanese Laid-Open Patent Application No. 2003-200043). In this method, an organic EL light-emitting sheet that emits ultraviolet light or visible light having a short wavelength is used as a light source to excite the photocatalyst. For example, two sheets are stacked, and the hazardous components in the fluid between the two sheets are decomposed and eliminated by photocatalytic action. However, in order to treat a large quantity of fluid with this structure, the number of sheet layers must be increased and numerous channels must be created, and the apparatus itself becomes extremely large.
Demand has recently increased for ceramic filters having high heat resistance, high strength, and high permeability. Such ceramic filters are used in the food and chemical industries. Organic films have been used in these industries in the past, but ceramics have excellent heat resistance, pressure resistance, chemical resistance, and high functionality not found in organic films, and ceramics have been replacing organic films. Furthermore, ceramic filters are used as catalyst carriers, microbial culture carriers, and other such bioreactors and the like.
Commonly used ceramic filters have a cross section shaped as a lotus root, wherein multiple channels are formed perpendicular to a cross section, and filtration layers are formed on the inner walls of the channels. Permeability performance is improved in actual practice by reducing the thickness of the narrow-pore filtration layer portions necessary for filtration. Specifically, the structure is composed of filtration layers for performing filtration, and support members for supporting the filtration layers. Ceramic filters whose cross sections are about 30 mm in diameter and about 500 to 1000 mm in length are often used. Overall, the porosity is about 35 to 40%, the pore diameters of the filtration layers are about 0.005 to 1 μm, and the pore diameters of the intermediate layers and support members are about 2 to 3 μm and 10 to 20 μm, respectively. The total thickness of a filtration layer and an intermediate layer is about 100 to 200 μm. A feed solution is poured into the channels and filtered by the filtration layers, and the clarified liquid passes through the intermediate layers and the support members and is ejected from the side of the ceramic filter.
However, this type of ceramic filter is not capable of physical filtration based on the relationship between the pore diameter and the size of the collected substances.
In contrast to this method, a ceramic filter has been invented that functions so that a light emitter and electrodes themselves are fashioned into a porous structure, the porous structure itself emits ultraviolet light when fluid passes through the structure, and the photocatalyst supported in the porous structure decomposes organic matter or destroys bacteria and viruses (see International Application Publication Pamphlet No. 04/006969). The light emitter is obtained by sintering some semiconductor particles to a certain degree.
However, the following problems are encountered with this method.
(1) An advanced technique is needed to control the pore diameter and porosity of the porous light-emitting layer. Particularly, in the case of an air purifier or the like which requires high permeability, the pore diameter and porosity must be increased and large semiconductor particles must be used, but sintering declines when the particle diameter is increased. Also, a light-emitting layer having a high porosity is difficult to obtain by a powder-sintering method.
(2) Costs are higher because an electrode must be formed on the surface of the porous structure by sputtering or vapor deposition.
(3) When a liquid, particularly a highly conductive liquid, passes through the interior of the porous light-emitting layer, an electric field sometimes cannot be effectively applied if the electrode and the particles constituting the porous light-emitting layer are not completely insulated, and an advanced technique is required for this insulation process. In particular, an even more advanced technique is required and costs are incurred when the constituent particles are reduced in size.